The major thrust of the research program is aimed at describing modes of control of blood flow in the skin of newborn growing rats. Arterioles in the skin of the newborn bat exhibit considerable tone. The first three projects are aimed at elucidating the nature of this tone, whether it is myogenic, due to sympathetic discharge or due to circulating catecholamines. The microvascular portion of the arterial tree, clear down to terminal arterioles is preformed at birth, and capillary networks may also be preformed. Studies are designed to evaluate this possibility. The celloid osmotic pressure of plasma is probably substantially lower in the newborn than in the adult animal, otherwise normal renal function could not be present. We plan to measure the plasma protein ontent in the newborn and growing animal. The resistance to flow in a given arteriolar bed increases with growth. This increased resistance is most likely due to gradually emerging precapillary sphincter activity. We propose to evaluate this possibility. The question has also emerged whether sympathetic vasoconstrictor nerve activity is an important mode of control in the adult bat wing. We propose to examine this by studying larger arterioles in the bat wing. A separate study is aimed at determining whether there is a relationship between sweat gland secretion and arteriolar blood flow. In recent years there has been an increasing awareness that tissue fluid onoctic pressures are high, in the order of 10-11 mmHg. A second series of studies is aimed at describing interactions between interstitial macromolecules such as collage, mucopolysaccharides, and plasma proteins. Plasma proteins, per se, are not present in sufficient concentration in the interstitium to account for the high oberseved oncotig pressures. We plan to "dissect" the tissue factors in the Starling equation in terms of volume exclusion of collagen and mucopolysaccharides as well as the direct osmotic contributions of mucopolysacchrides and plasma proteins. These data are essential for predicting volume shifts of interstitial fluid resulting from disturbances in capillary fluid balance.